This invention relates to a system for detecting actual air/fuel ratio in an internal combustion engine by using an oxygen sensor in the exhaust gas, the oxygen sensor being of the concentration cell type requiring the supply of a small DC current thereto for the purpose of producing and maintaining a reference oxygen partial pressure therein.
In recent internal combustion engines and particularly in automotive engines, it is prevailing to control the air/fuel mixing ratio precisely to a predetermined optimum value by performing feedback control with a view to improving the efficiencies of the engine and reducing the emission of noxious or harmful substances. In the current feedback control systems for this purpose it is usual to produce a feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine by sensing the concentration of oxygen in the exhaust gas since there is a determined relationship between the air/fuel ratio in the engine and the oxygen content in the exhaust gas. In many cases the target value of the air/fuel ratio under feedback control is a stoichiometric air/fuel ratio. For example, when a so-called three-way catalyst is used in the exhaust system to achieve reduction of NOx and oxidation of CO and HC simultaneously, the air/fuel ratio must be controlled precisely to a stoichiometric ratio because this catalyst exhibits highest conversion efficiencies in an exhaust gas produced by combustion of a stoichiometric air-fuel mixture.
As for the device to sense the oxygen concentration in the exhaust gas to thereby detect actual air/fuel ratio in the engine, it is usual to use an oxygen sensor of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte such as zirconia containing a small amount of stabilizing oxide such as yttria, a measurement electrode layer formed porously on an outer side of the solid electrolyte layer and a reference electrode layer formed on the opposite side. In an oxygen sensor of this type it is necessary to maintain a reference partial pressure of oxygen on the reference electrode side of the solid electrolyte layer, and it is well known to expose the reference electrode layer to air as the source of reference partial pressure of oxygen by forming the solid electrolyte layer into a tubular shape. This oxygen sensor generates an electromotive force of which the magnitude depends on the difference between the partial pressure of oxygen in a gas coming into contact with the measurement electrode layer and the reference oxygen pressure at the reference electrode layer. In the exhaust gas of an internal combustion engine the output of this oxygen sensor exhibits a great and sharp change when the air/fuel ratio in the engine changes across the stoichiometric point because such a change in the air/fuel ratio results in a great change in the content of oxygen in the exhaust gas. However, the tubular shape of the solid electrolyte layer to expose only the reference electrode to air offers various problems such as low productivity of the oxygen sensor and difficulty in desirably reducing the size of the sensor.
Recently an advanced oxygen sensor of the concentration cell type has been developed as shown in U.S. Pat. No. 4,207,159 for example. The sensitive part or oxygen concentration cell of this oxygen sensor takes the form of a laminate of very thin layers, including a porous solid electrolyte layer and two electrode layers, supported on a plate-shaped ceramic substrate of very small size. As a feature of this sensor, a reference partial pressure of oxygen is produced and maintained in the laminated cell without using any extra oxygen source material by supplying a very small DC current to the cell so as to flow in the solid electrolyte layer between the two electrode layers. The flow of the current causes migration of oxygen ions in the solid electrolyte layer toward the reference electrode layer, and at this electrode the oxygen ions are converted to oxygen molecules, which gradually diffuse outward through the micropores in the solid electrolyte layer. Consequentially a nearly constant partial pressure of oxygen as a balance between the migration of oxygen ions and the outward diffusion of oxygen molecules is maintained at the reference electrode while the measurement electrode layer is exposed to an oxygen-containing gas subjected to measurement. In the exhaust gas, this oxygen sensor too exhibits a great and sharp change in the level of its output voltage in response to a change in the air/fuel ratio in the engine across the stoichiometric ratio, though it is also possible to modify the output characteristic of this oxygen sensor as shown in U.S. Pat. No. 4,224,113 for example.
This advanced oxygen sensor is advantageous in many respects such as quickness of response, smallness in size and high productivity and therefore has been increasingly put into industrial practice. However, there is the need to adequately control the current supplied to this oxygen sensor because the rate of diffusion of oxygen molecules in the porous solid electrolyte layer as a factor in producing the reference oxygen partial pressure varies depending on the temperature of and oxygen content in the exhaust gas and such properties of the exhaust gas are variable depending on the operating conditions of the engine. Particularly it is undesirable that the reference oxygen partial pressure becomes excessively high as the result of unbalance between the rate of supply of oxygen ions to the reference electrode layer by the action of the current and the rate of the outward diffusion of oxygen molecules, because there arises a possibility that the thin reference electrode layer or the laminated cell as a whole peels away from the ceramic substrate by the action of the excessively increased pressure of oxygen.